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We report detailed measurements of transport and electronic properties of molecular tunnel junctions based on self-assembled monolayers (SAMs) of oligophenylene monothiols (OPTn, n = 1-3) and dithiols (OPDn, n = 1-3) on Ag, Au, and Pt electrodes. The junctions were fabricated with the conducting probe atomic force microscope (CP-AFM) platform. Fitting of the current-voltage (I-V) characteristics for OPTn and OPDn junctions to the analytical single-level tunneling model allows extraction of both the HOMO-to-Fermi-level offset (ϵ h ) and the average molecule-electrode coupling (Λ) as a function of molecular length (n) and electrode work function (Φ). Significantly, direct measurements of ϵh UPS by ultraviolet photoelectron spectroscopy (UPS) for OPTn and OPDn SAMs on Ag, Au, and Pt agree remarkably well with the transport estimates ϵh trans , providing strong support - beyond the high quality I-V simulations - for the relevance of the analytical single-level model to simple molecular tunnel junctions. Because the UPS measurements involve SAMs bonded to only one metal contact, the correspondence of ϵh UPS and ϵh trans also indicates that the top contact has a weak effect on the HOMO energy. Corroborating ab initio calculations definitively rule out a dominant contribution of image charge effects to the magnitude of ϵ h . Thus, the effective molecular tunnel barrier ϵ h is determined, and essentially pinned, by the formation of a single metal-S covalent bond per OPTn or OPDn molecule.
Bibliographical noteFunding Information:
C.D.F. acknowledges financial support from the U.S. National Science Foundation (CHE-1708173). Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. I.B. acknowledges financial support from the Deutsche Forschungsgemeinschaft (DFG grant BA 1799/ 3-1), and computational support from the State of Baden-Württemberg through bwHPC and the DFG through Grant No. INST 40/467-1 FUGG.
© 2019 American Chemical Society.
PubMed: MeSH publication types
- Journal Article
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